Generally a conventional color CRT comprises a screen panel 21, a fluorescent layer 24 formed on the inner side of the screen panel, an electron gun 26 and a shadow mask 25 installed between the electron gun and the fluorescent layer. The shadow mask is supported on the mask frame together with an earth magnetic field shield, a resilient member 22, etc.
The shadow mask 25 has several hundred thousand electron apertures whose total area is less than one third of the effective area of the shadow mask. About two thirds of the electrons emitted from the electron gun 26 do not pass through the apertures and strike blocked area around the apertures to heat the shadow mask 25 to a temperature of more than about 80.degree. C. Consequently, the shadow mask 25 undergoes a thermal expansion causing the central part to bow towards the fluorescent layer 24, which bowing phenomena is generally referred to as doming, so that the normal position of the apertures of the shadow mask 25 deviates. For this reason, the electrons emitted from the electron gun 26 pass through the apertures of the distorted position mislanding on incorrect spots of the fluorescent layer 24. As a result the images are overlapped together on the screen resulting in unclear pictures.
Therefore, as disclosed in Japanese Patent Publication No. 44-3547, the conventional color CRT is provided with a bimetal between the inner wall of the screen panel 21 and the outer wall of the mask frame 27 to maintain the distance between the shadow mask 25 and fluorescent layer 24 within a given range. FIG. 2 shows that the mislanding due to doming of the shadow mask 25 is being corrected by such a bimetal.
In this case, the first interval indicates the extent of mislanding owing to the position of the apertures deviated by the heating of the shadow mask 25, and the second interval indicates the mislanding extent is compensated by the bimetal. In FIG. 3 and 4, the position of the shadow mask 25 at the turning-on point (t=0) of the color CRT is 5a and the deviated position owing to the increasing temperature of the shadow mask 25 is 5b in the first interval. Accordingly the route of an electron beam passing an aperture of the shadow mask 25 is changed from C1 to C2. As a result, the electron beam is mislanded on the fluorescent layer 24 by the extent of .DELTA.x. Such a mislanding is compensated by the thermal expansion of a bimetal by the heat transferred from the shadow mask 25 through the mask frame 27. Since this compensation occurs after the time t=T as shown in FIG. 2, the conventional method of using a bimetal takes a long compensation time.